1. Field of the Invention
The present invention relates to an electronic ballast for the operation of at least one gas discharge lamp.
2. Description of the Related Art
If a plurality of electronic ballasts for the operation of gas discharge lamps are wired together to form a larger illumination system, normally the phase terminals and the neutral lines of the ballasts are first connected with one another and then connected to the mains. If the common connection of the neutral lines of the ballasts is not or is only insufficiently connected with the mains neutral line, or other faults arise in the wiring, there can arise at the ballasts an undefined voltage condition, with the consequence that the voltages lie above the normal supply voltage in the region between 220 and 380 volts.
This overvoltage brings with it the danger that the apparatuses could be destroyed if the causes of the faulty wiring are not removed in a timely manner. Although there would be the possibility of so configuring the ballasts that they self-actingly compensate the overvoltage, through this however a not inconsiderable amount of power would be consumed to no end. For this reason, the ballasts are as a rule so configured that in the event of overvoltages they work for at least a limited period of time. In this case, however, there is then necessary a recognition of the overvoltage condition at the earliest possible time, in order to be able to take suitable steps.
The present invention is thus based on the object of so configuring an electronic ballast for the operation of at least one gas discharge lamp that it makes possible a recognition of fault conditions in the voltage supply.
This object is achieved by means of an electronic ballast which has an overvoltage monitoring device, which monitors the supply voltage delivered to the electronic ballast and, in the event that the supply voltage exceeds a predetermined nominal value, initiates an optically or acoustically perceptible warning signal. The ballast is thus itself not in a position to act to counter the overvoltage. Instead of this it is ensured that the fault in the voltage supply is indicated directly after the switching on of the lamp, so that appropriate measures can be taken to remove the overvoltage. By means of the measures in accordance with the invention the operational security of the ballast is thus significantly increased.
Further developments of the invention are also described and claimed herein. For example, an input signal is delivered to the overvoltage monitoring device. This input signal preferably may be the supply voltage rectified by a rectifier circuit on an input side of the ballast. The detected signal is then compared, with the aid of a comparator, with a nominal value. Alternatively to a simple comparator, there may, however, also be provided an analogue/digital converter, which converts the detected measurement value into a digital value consisting of at least two bits, through which the exactitude of the overvoltage monitoring is increased.
Preferably, upon recognition of an overvoltage condition, the overvoltage monitoring device sends a corresponding signal to a control circuit which is provided for the control of the inverter of the ballast, whereby the control circuit, as reaction to the received overvoltage signal, periodically switches the lamp on and off or periodically modulates the brightness of the lamp. A particularly elegant development of the invention consists in generating the periodic brightness variation of the lamp in that the regulation circuit for the operation of the inverter is influenced. For example, if the control circuit detects an operating parameter of the load circuit, compares this with a reference value, and generates in dependence upon the comparison result control signals for controlling the inverter, it can thus be provided that the reference value of the regulation circuit is periodically altered. Through the alteration of the reference value, the lamp is automatically operated with a modulated brightness, without particular measures being necessary for this purpose.
A further development of the invention incorporates a digital regulation circuit. This involves providing, in the control circuit, an analogue/digital converter for converting the detected operating parameter of the load circuit into a digital value consisting of at least two bits, whereby the control circuit calculates on the basis of this digital value, in a digital regulation circuit, switching information for the operation of the inverter and passes this on to a driver circuit which converts the switching information into corresponding control signals for the operation of the inverter. The analogue/digital converter may then operate in time multiplex mode and be employed at regular intervals also for the detection and the evaluation of the supply voltage. This variant makes possible an integration of the overall ballast to the greatest possible extent, since the components of the control circuit are integrated into a single circuit and at the same time can be used also for realisation of the overvoltage monitoring. In particular it can be provided that the control circuit is constituted as an application specific integrated circuit (ASIC).